An optical fiber core wire is generally composed of a glass optical fiber made of a silica glass or the like, and a covering applied on the outer periphery of the glass optical fiber and having a two-layer structure composed of a soft layer and a hard layer, wherein the covering is made of an ultraviolet curable resin, a thermo-setting resin, or the like. The soft layer (hereinafter referred to as a primary covering layer) contacting with the glass optical fiber is made of a soft resin having a comparatively low Young's modulus and functions as a buffer layer (stress relaxation layer). The hard layer (hereinafter referred to as a secondary covering layer) situated on the outer periphery of the primary covering layer is made of a hard resin having a comparatively high Young's modulus and functions as a protection layer. Further, a very thin colored layer for identification is sometimes applied on the outer periphery of the secondary covering layer as the occasion demands.
Generally, as the primary covering layer, a resin having a Young's modulus of 3 MPa or less after coating is used, and as the secondary covering layer, a resin having a Young's modulus of 500 MPa or more is used.
Various types of optical fiber cables having a sheath integrally covering the optical fiber core wire and at least one tension member provided parallely to the optical fiber core wire on one side or both sides of the optical fiber core wire are being manufactured and used. Moreover, when such an optical fiber cable is aerially laid, a supporting wire covered with a sheath is generally attached in the lengthwise direction of the optical fiber cable.
When such optical fiber cables are aerially laid, degradation in their characteristics of an unknown origin occasionally occurs as time elapses. The cause of this degradation in their characteristics has recently proved to be the ovipositional behaviors of cicadae into the optical fiber cables, which develop in summer, especially Cryptotympana facialises.
In particular, the cause is the behavior of a Cryptotympana facialis to misjudge an aerially laid optical fiber cable to be a trunk or a branch of a tree and thrust the ovipositor of the Cryptotympana facialis into the sheath of the optical fiber cable to lay eggs into the optical fiber cable.
If the ovipositor is thrust into the sheath in such a way, the ovipositor is thrust into the inner optical fiber damaging the optical fiber, and the optical fiber is disconnected.
Accordingly, various methods for preventing the disconnecting of optical fibers due to the ovipositional behaviors of cicadae and the like have been proposed. For example, the method of forming slit-shaped notches wherein the tip lines reach the neighbor of an optical fiber at almost diagonal positions with the optical fiber core wire put between the positions on the surface of the sheath as described in Japanese Patent Application Laid-Open Publication No. 2006-163337 and the method of forming a low friction and high strength resin composition layer on the surface of a sheath as described in Japanese Patent Application Laid-Open Publication No. 2007-101586.